In this project, we propose to develop and demonstrate feasibility of a smartphone-based mobile eye alignment application (MEAA) for use by lay people as well as eye-care professionals to screen for strabismus. Strabismus develops in an estimated 3-8% of the U.S. population, and is a leading risk factor for amblyopia in young children. Early detection and treatment are critical to prevention of permanent vision losses, yet only 40% of U.S. children under age 6 reportedly receive vision screening. In the elderly and in head trauma populations where the prevalence is highest, untreated strabismus can result in postural instability, leading to a significantly greater risk of devastating falls. However, signifcant barriers of cost and availability of trained examiners limit access to strabismus screening for at-risk populations, especially in under-served rural and low-income areas and elder care facilities. The novel MEAA, currently capable of resolving 3 prism diopter differences in alignment (equivalent to 1.7 angle), provides the first image-based tool for eye alignment testing by lay persons without reliance on a costly device. We propose to further develop the MEAA to enable accurate detection of strabismus in a wide variety of subjects, including individuals wearing corrective lenses, with vertical strabismus, with non-standard iris diameter, and subjects who exhibit anomalous head posture or are unable to maintain fixation. Through our clinical collaborations we will evaluate MEAA performance relative to standard test methodology in targeted populations likely to contain subjects who are difficult to accurately measure, including both young children and head trauma patients. We will also report initial estimates of MEAA sensitivity and specificity in these targeted populations. At the conclusion of the Phase I project we will have demonstrated feasibility of the novel mobile eye alignment application, showing accuracy, precision, and reproducibility comparable to or exceeding costly tests now employed in strabismus screening programs outside ophthalmology practices. The wide availability of this low cost, smartphone-based strabismus screening application could make possible a paradigm shift in cost-benefit calculations and increase awareness for vision screening programs, resulting in greater access to treatment for strabismus-affected individuals. Once the sensitivity and specificity for the MEEA meet or exceed those of current methods in populations representative of vision screening, we will prepare for clinical trials and FDA/IDE submission in Phase II.